1. Field of the Invention
The present invention concerns a non-spherical surface measuring device that is used to measure, with a high degree of accuracy, the surface precision of non-spherical surfaces which have a rotationally symmetrical axis.
2. Discussion of the Related Art
The object surfaces to be examined and measured are surfaces that have a so-called higher-order non-spherical surface design shape in which the non-spherical surface shape can be expressed by Equation (1) as follows: ##EQU1## In Equation (1), it is not necessary that the higher-order non-spherical surface coefficients C.sub.i be limited to 10 orders, and there is no loss of generality in the description that follows. Furthermore, the coefficient .kappa. indicates a conic coefficient and the coefficient R indicates the central curvature radius. In order to simplify the equation it is written in a notation which differs from the ordinary notation in that "1+.kappa." in the ordinary notation is replaced by ".kappa.". Thus, for example, a parabolic surface, which is expressed as ".kappa.=-1" in ordinary notation, is expressed as ".kappa.=0" in the present specification.
Furthermore, in the present specification, the higher-order non-spherical surface coefficients C.sub.02 through C.sub.10, in Equation (1), are defined as higher-order terms. In regard to the second-order term with C.sub.02 as a coefficient, since this is equivalent to .kappa.=0 (when .kappa.=0, the first term on the right side of Equation (1) and the second-order term which has C.sub.02 as a coefficient are equivalent), C.sub.02 may be omitted in some cases when a higher-order non-spherical surface shape is expressed. However, since higher-order non-spherical surfaces, which use a conical surface for which .kappa. does not equal 0 as a base, and which also include components of the C.sub.02 term, C.sub.02 will be included in the higher-order terms in the present invention. In this regard as well, there is no loss of generality in the following description.
It is known that where the components of the higher-order terms are small (i.e., in the case of micro-higher-order non-spherical surfaces), so-called "folded back null measurement" is possible. Generally, in interference measurements, it is known that apparent aberration, caused by alignment, cannot be avoided and that so-called alignment error correction is therefore necessary as a treatment for eliminating such aberration when high-precision measurements are to be performed.
Where higher-order terms cannot be ignored, or more precisely, where the number of interference fringes is so great that high-precision measurement is impossible, such as folded back null measurement, a higher-order non-spherical surface measurement method is used in which a null wave front is generated by means of a null lens or a zone plate element and interference measurements are performed without using the folded back configuration.
Currently, however, no appropriate method for correcting alignment error in the conventional measurement of higher-order non-spherical surfaces has been proposed. Also no appropriate method for the measurement of the wave front shape of the null wave front itself used in the measurement of higher-order non-spherical surfaces has been proposed.
For example, lenses with high-precision higher-order non-spherical surfaces are used in the exposure processes of semiconductor manufacturing processes. The method used to manufacture such high-precision lenses consists of a process in which highly uniform glass elements, which meet the required specifications, are cut to a prescribed size. The process further includes a grinding process, in which the cut elements are ground to a shape that is close to the final shape, a polishing process, in which the ground surfaces are finished to form polished surfaces. If the surface precision remains insufficient, a corrective polishing process is performed in accordance with measurement results. The measurement method used is basically an interferometer constructed as shown in FIG. 2. The polished non-spherical surface that is to be measured is set in a prescribed holding position relative to the interferometer and the surface precision is measured from the condition of the interference fringes produced. In this case, however, there is no guarantee that the object of measurement is set in the prescribed position. Specifically, since the measurement results conceivably include an alignment error, alignment is repeated and the 3D diagram obtained when the "RMS value calculated, as a result of measurement" reaches a minimum, is taken as the surface precision of the workpiece.
Repeated measurement is necessary in order to ascertain whether or not the value is actually the minimum value, so that the measurement time required is increased. Objective measurement is impossible, since there is a possibility that the measured values will differ depending on the technical skill of the measurer. Additionally, if a high detected surface precision is required, there are limits in terms of precision of the positioning that can be accomplished by hand.
Thus, time-consuming adjustment is required in order to reduce the alignment error of the lens and satisfactory measurement precision cannot be obtained even when such adjustment is performed. Accordingly, there have been major problems in the area of lens manufacturing time and lens performance.